Aperiodic magnetic turbulence produced by relativistic ion beams

TL;DR

This study uses high-resolution PIC simulations to investigate magnetic turbulence generated by relativistic ion beams in plasmas, revealing aperiodic turbulence capable of efficient particle scattering and magnetic field amplification relevant to astrophysical shocks.

Contribution

It provides detailed simulation evidence of magnetic turbulence properties and saturation levels caused by relativistic ion beams, extending understanding of cosmic-ray driven instabilities.

Findings

Magnetic turbulence saturates at a few times the ambient magnetic field.

Aperiodic turbulence enables efficient particle scattering consistent with Bohm diffusion.

Non-resonant modes can amplify magnetic fields significantly beyond initial levels.

Abstract

Magnetic-field generation by a relativistic ion beam propagating through an electron-ion plasma along a homogeneous magnetic field is investigated with 2.5D high-resolution particle-in-cell (PIC) simulations. The studies test predictions of a strong amplification of short-wavelength modes of magnetic turbulence upstream of nonrelativistic and relativistic parallel shocks associated with supernova remnants, jets of active galactic nuclei, and gamma-ray bursts. We find good agreement in the properties of the turbulence observed in our simulations compared with the dispersion relation calculated for linear waves with arbitrary orientation of ${\vec k}$. Depending on the parameters, the backreaction on the ion beam leads to filamentation of the ambient plasma and the beam, which in turn influences the properties of the magnetic turbulence. For mildly- and ultra-relativistic beams, the instability saturates at field amplitudes a few times larger than the homogeneous magnetic field strength. This result matches our recent studies of nonrelativistically drifting, hot cosmic-ray particles upstream of supernova-remnant shocks which indicated only a moderate magnetic-field amplification by nonresonant instabilities. We also demonstrate that the aperiodic turbulence generated by the beam can provide efficient particle scattering with a rate compatible with Bohm diffusion. Representing the ion beam as a constant external current, i.e. excluding a backreaction of the magnetic turbulence on the beam, we observe non-resonant parallel modes with wavelength and growth rate as predicted by analytic calculations. In this unrealistic setup the magnetic field is amplified to amplitudes far exceeding the homogeneous field, as observed in recent MHD and PIC simulations.